Ferritic steels are attractive because of low cost, ease of fabrication, low thermal expansion and good thermal conductivity, but are limited by high-temperature strength and low-temperature toughness. For many modern applications, heat-resisting, structural steel alloys are required that have both high strength and high toughness. While it is desirable to produce a steel having both characteristics, in practice, the improvement in one characteristic usually comes at the expense of the other. Toughness is an intrinsic characteristic mechanical property typically determined by the upper-shelf energy (USE) and the ductile-brittle-transition temperature (DBTT) as measured by the Charpy impact test. The most desirable steels have a high USE and a low DBTT.
Ferritic steels, and in particular, Cr-Mo steels, have been proposed for use for the first wall and blanket structure of nuclear fusion reactors since these steels have been found to have excellent resistance to radiation-induced void swelling when irradiated in a fast fission reactor. See, for example, "Chromium-Molybdenum Steels for Fusion Reactor First Walls-A Review", by R. L. Klueh, Nuclear Engineering and Design 72 (1982 North-Holland Pub. Co.); "On The Saturation Of The DBTT Shift Of Irradiated 12Cr-1MoVW With Increasing Fluence", by J. M. Vitek, et al., Journal of Nuclear Materials 141-143 (1986); and "The Development of Ferritic Steels For Fast Induced-Radioactivity Decay For Fusion Reactor Applications", by R. L. Klueh and E. E. Bloom, Nuclear Engineering and Design/Fusion 2 (1985). A critical problem for nuclear fusion applications for such steels is that various alloying elements, including molybdenum, nickel, nitrogen, copper, and niobium, undergo transmutation reactions caused by irradiation from high-energy neutrons created by nuclear reactions in a fusion plasma. These elements, when used in alloys to make fusion reactor components, produce highly radioactive isotopes that decay over a long period of time. Thus, after the service lifetime of the reactor, deep geological disposal of the radioactive components becomes necessary.
To simplify waste disposal, new structural materials known as "low activation" or "reduced-activation" or "fast induced-radioactivity decay" (FIRD) alloys have been proposed. Such new alloys should at least meet guidelines issued by the U.S. Nuclear Regulatory Commission (10 CFR Part 61) for shallow land burial, instead of the much more expensive deep geologic disposal. Decay to low radioactivity levels for such FIRD alloys would occur in tens of years instead of the hundreds or thousands of years required for conventional steels. Thus, FIRD alloys must not contain molybdenum or other alloying elements which produce long-lived radioactive isotopes when used in nuclear fusion applications.
The need for both strength and toughness is also important for nuclear fission and non-nuclear, elevated-temperature structural heat-resisting steel applications. A low DBTT is required because steels can become embrittled by an increase in the DBTT after prolonged exposure to elevated temperatures. Therefore, low-chromium steels having high strength and toughness will have many non-nuclear applications, such as in power generation systems or chemical reaction vessels, where the 2.25Cr-1Mo ferritic steel is used extensively.
Three commercial Cr-Mo steels presently available for non-nuclear applications are 2.25Cr-1Mo (Fe-2.25%Cr-1%Mo-0.1%C), 9Cr-1MoVNb (Fe-9%Cr-1%Mo-0.25%V-0.07%Nb-0.1%C), and 12 Cr-1MoVW (Fe- 12%Cr-1%Mo-0.25%V-0.5%W-0.5%Ni-0.2%C) wherein all concentrations are in weight percent. The molybdenum, niobium, and nickel content keep these commercial steels from being FIRD steels for nuclear fusion applications. The 9Cr-1MoVNb and 12Cr-1MoVW steels have better elevated temperature strength and oxidation resistance than 2.25Cr-1Mo steel. However, the relatively high concentration of chromium in these steels is not desirable, particularly for fusion reactor applications, due to their relatively poor weldability. Also, since chromium is expensive and a strategic element of uncertain supply, steels requiring less chromium would naturally be desirable.
Cr-W steels have been considered for making structural components of fusion reactors, including the following alloys: 2.25Cr-2W; 2.25Cr-2WV; 2.25Cr-1WV; 2.25CrV; 9Cr-2WVTa; and 12Cr-2WV. Properties of these steel alloys are discussed in "Impact Behavior of Cr-W Steels", by R. L. Klueh and W. R. Corwin, J. Materials Engineering, Vol. 11, No. 2 (1989); and "Heat Treatment Behavior and Tensile Properties of Cr-W Steels", by R. L. Kleuh, Metallurigcal Transactions A, Vol. 20A, March 1989. The 9Cr-2WVTa described therein had the best combination of strength and toughness. The 2.25Cr-2WV steel had the best strength, but toughness was poor, thus making it unsuitable for fusion applications. It was concluded that the reason for the high DBTT for the 2.25Cr-2WV involved the low hardenability of the steel, which leads to the steel having a duplex structure of bainite and polygonal ferrite after normalization as 15.9 mm thick plate, compared to the 2.25Cr-2W, which was 100% bainite and has a lower DBTT. However, even when the 2.25Cr-2WV was heat treated to produce 100% bainite by cooling thin sections, it still did not exhibit the good toughness of the 2.25Cr-2W and 9Cr-2WVTa steels. The 2.25Cr-2W steel was determined to be less attractive because of its low strength.